Magnetic bubble or single wall domain memories are now well known in the art. The most familiar mode of operating a bubble memory, called the "field-access" mode, utilizes a pattern of magnetic elements, typically of a magnetically soft material, in a plane next to the plane of the film in which bubbles move. The elements respond to a magnetic drive field rotating in the plane of bubble movement to produce field gradients in localized areas occupied by a pattern of bubbles. The gradients effect bubble movement in the film along channels defined by the pattern of elements.
One organization for a field-access mode bubble memory is called the "major-minor" organization. This type of organization requires that the magnetically soft elements (typically of permalloy) be arranged to form a plurality of closed loop (minor) paths for recirculating bubble patterns and a single (major) accessing channel to and from which bubble patterns are transferred for write and for read operations.
In a typical prior art multiple track memory, such as a disc file, binary words, each including data and address bits, are stored in sequence and passed by read-out heads. A selected address is applied to a comparator in a manner to provide an output only when a matching one of the sequences of addresses is applied to the comparator. In single track sequential memories, data and address bits are stored in sequence but are accessed in similar fashion. When a selected address of the sequence is found, the associated data is read out. Here, however, both data and addresses must pass the read-out head sequentially resulting in a relatively slow access time.
In the major-minor bubble memory, data moves in two directions in a plane. Consequently, there is no direct comparison between this operation and either of the multiple or single track memories. Nevertheless, the movement of data in a bubble memory from successive positions in the minor loops to the accessing (major) channel results in a sequence of data bits being compiled in the major channel and moved to a read-out position. If this data comprises address bits as well as data bits, a search-type major-minor bubble memory would have to look at all bits in memory sequentially (both data and address bits) in order to find a selected address. The access time would be slow in this case. A problem, therefore, is to provide a bubble memory operative in a search mode yet characterized by a relatively fast access time.
Information in a bubble memory, further, is often tracked by a housekeeping loop as is well known. A power failure at an inopportune time could result in the scrambling of information rendering the store inoperative unless data is rewritten. A variety of implementations is known for dealing with such a possibility. But these typically require special electronics for their realization.